Artificial nuclides with atomic number of 93 but with different mass numbers
Isotopes of neptunium (93Np)
Main isotopes[1]
Decay
abundance
half-life (t1/2)
mode
product
235Np
synth
396.1 d
α
231Pa
ε
235U
236Np
synth
1.54×105 y
ε
236U
β−
236Pu
α
232Pa
237Np
trace
2.144×106 y
α
233Pa
239Np
trace
2.356 d
β−
239Pu
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Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238 U with neutrons to produce 239 U , which then underwent beta decay to 239 Np .
Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.[2]
Twenty-five neptunium radioisotopes have been characterized, with the most stable being 237 Np with a half-life of 2.14 million years, 236 Np with a half-life of 154,000 years, and 235 Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has five meta states, with the most stable being 236m Np (t1/2 22.5 hours).
The isotopes of neptunium range from 219 Np to 244 Np , though the intermediate isotope 221 Np has not yet been observed. The primary decay mode before the most stable isotope, 237 Np , is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237 Np are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Neptunium is the heaviest element for which the location of the proton drip line is known; the lightest bound isotope is 220Np.[3]
^Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
^Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. (1952). "Occurrence of the (4n + 1) series in nature" (PDF). Journal of the American Chemical Society. 74 (23): 6081–6084. doi:10.1021/ja01143a074.
^Zhang, Z. Y.; Gan, Z. G.; Yang, H. B.; et al. (2019). "New isotope 220Np: Probing the robustness of the N = 126 shell closure in neptunium". Physical Review Letters. 122 (19): 192503. Bibcode:2019PhRvL.122s2503Z. doi:10.1103/PhysRevLett.122.192503. PMID 31144958. S2CID 169038981.
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